Electrical on-board networks of the general type mentioned above are available. For example, European Patent No. EP 1 646 522 B1 describes a motor vehicle on-board network having a multi-voltage generator motor that includes an electrical machine, a regulator, and a pulse inverter. These are provided for the supply of electrical power to a first sub-network having at least one first consumer and to a second sub-network having at least one second consumer. In the first sub-network, a first nominal voltage should be present, and in the second sub-network a second nominal voltage should be present, and a DC/DC converter should be situated downstream from the multi-voltage generator motor. Both the first and the second sub-network are connected to the multi-voltage generator motor via the pulse inverter. Using the described motor vehicle on-board network, it is possible to supply the first sub-network with a variable voltage.
If the electrical machine is fashioned as a starter-generator, in particular a belt-driven starter-generator, it can be used both in order to recuperate brake energy in a recuperation operating mode and to realize a stop-start function. In the latter case, for example a drive aggregate of the drive of the vehicle, in particular an internal combustion engine, is switched off during periods of time in which it is not required. If the power of the internal combustion engine is to be made available again, the electrical machine is used to start the internal combustion engine. If the vehicle is a microhybrid vehicle, the energy required for the starting is provided for example by a conventional lead accumulator. This has the disadvantage that it has only a limited maximum charge current or discharge current, which in addition is still a function of its state of charge. Thus, the possibility of saving energy through recuperation of braking energy, and thus reducing the CO2 emission of the drive, is significantly limited.
For this reason, as described in European Patent No. EP 1 646 522 B1, the electrical machine is used together with a capacitor and a DC/DC converter. Here, with the aid of the DC/DC converter the energy recuperated during the braking process is stored in the capacitor and/or is provided to one of the power circuits. When the capacitor is discharged, the DC/DC converter can in addition be used to charge the capacitor with energy from the accumulator. In this way, the vehicle can easily be fashioned as a microhybrid, micro-mild hybrid, or mild hybrid. The disadvantage of such an electrical on-board network is however its comparatively high cost outlay.